1. Field
This invention relates to steam generators and more particularly to methods and apparatus for manipulating equipment around the secondary side of a tube sheet of steam generators.
2. Description of Related Art
A nuclear steam generator is a pressurized vessel divided into a primary and a secondary side. The primary and the secondary sides are separated by the “tube sheet”. As in any heat exchanger, both primary and secondary sides have an inlet and an outlet. In order to increase the heat exchange surface, the tube sheet is drilled with a plurality of holes organized in two groups. The primary side is divided in two sections by a “divider plate” in a way that one group of holes communicates with the primary side inlet (to form the “hot leg”) and the second group of holes communicates with the primary side outlet (to form the “cold leg”). U-shaped tubes attached to the tube sheet extend in the secondary side and connect the holes from the hot leg to the holes from the cold leg. These U-shaped tubes form the tube bundle. The primary hot water can now enter the hot leg, travel through the tubes where the heat transfer takes place and leave the steam generator through the cold leg. On the secondary side, relatively cold water (“feedwater”) enters through the secondary side inlet (“feed water nozzle”), turns into steam from the heat transfer through the tubes and the steam exits through the secondary side outlet (“steam nozzle”). This configuration is described, for example, in U.S. Pat. Nos. 8,238,510; 5,036,871; 4,273,076; and 4,079,701 (Haberman; Ruggieri et al.; Lahoda et al. and Hickman et al., respectively), many of which relate to top of tube sheet sludge removal.
Since the primary fluid contains radioactive particles and is isolated from the feedwater only by the U-tubes, the U-tube walls are the boundary for isolating these radioactive particles from the secondary side. It is, therefore, important that the U-tubes be maintained defect-free so that no leaks/breaks will occur in the U-tubes.
A variety of degradation mechanisms have been experienced on the shell side of steam generators, i.e., the secondary side. These degradation mechanisms may be loosely divided into two categories; mechanical degradation, such as wear or denting and chemical induced degradation such as stress corrosion cracking (SCC) or Inter/transgranular attack. High caustic levels found in the vicinity of the cracks in tube specimens taken from operating steam generators and the similarity of these cracks to failures produced by caustic environments under controlled laboratory conditions, have identified high caustic levels as the possible cause of the intergranular corrosion, and thus the possible cause of the tube cracking. Acidic conditions have also empirically demonstrated the ability to cause tubing degradation. Elevated concentrations of deleterious species such as lead or copper and conditions with an elevated electrochemical potential are also catalysts for tubing accelerated degradation as a result of localized mechanical stresses from deformation of tubing via in situ formation of magnetite, known as denting. These degradation mechanisms typically occur in the vicinity of a sludge pile present on the top of tube sheet on the shell side of the steam generator. The sludge is mainly iron oxide particulates and copper compounds along with traces of other minerals that have settled out of the feedwater onto the tube sheet, and into the annulus between the tube sheet and the tubes. The level of sludge accumulation may be inferred by eddy current testing with a low frequency signal that is sensitive to the magnetite in the sludge. The correlation between sludge levels and the tubing degradation location strongly suggests that the sludge deposits provide a site for concentration of impurities at the tube wall that results in the onset of tubing degradation. Loose parts within the secondary side can also result in tube wall degradation and can also settle out on top of the tube sheet.
To remove these deposits, sludge lancing and inspections are performed every one to two refueling outages. Currently, standard practice involves spraying high pressure water through the tube bundle and directing the flow to suction hoses where the loose deposits can be removed and filtered. These suction hoses may be located at a substantial distance from the completely separate high pressure lance. This prior art process typically requires large pumping and filtration systems which use several hoses to deliver the cleaning media, which can be located over 500 feet away. The high pressure water is typically delivered from the “no” (central) tube lane (lane without tubes separating the hot leg side from the cold leg side of the tube bundle under the U-bend region) of the steam generator and “pushes” the deposits into the suction hose system. The lancing process requires the tube sheet to be lanced several times to ensure satisfactory cleanliness results, which is time consuming and not cost effective.
In most nuclear steam generators in service today, there are usually 6 inch (15.2 cm.) diameter hand holes in the shell of the steam generator near and above the tube sheet that has an associated hole in the wrapper providing access to the tube sheet for removal of the sludge deposits.
In regard to the description of the related art set forth above, there is a need for a method and apparatus that can effectively clean and remotely inspect the top of the tube sheet of a steam generator with a relatively low cost and high efficiency, without requiring multiple passes to obtain a satisfactory result. Accordingly, it is a main object of this invention is to provide such a method and apparatus.